1. Field of the Invention
The present invention relates to a corona apparatus for the surface treatment of melts or films of thermoplastic materials or of metal foils.
2. Description of the Prior Art
The surface treatment of plastic materials or metals by means of corona devices is well known in the prior art.
Thus, films of thermoplastic materials or metal foils, e.g. aluminum foils, are subjected to electric corona discharges in order to render them more easily printable, or to improve their adhesion to layers applied by laminating or film coating processes, or to improve the cohesion of a composite material towards coatings applied from solutions or dispersions.
In most cases, the corona apparatus is so constructed that the materials to be treated are guided over a supporting surface, normally a roller, which acts as the counter electrode and is grounded, and that one or more electrodes are arranged above the supporting surface. By means of a generator, high-voltage alternating current is generated in the electrodes; more recently, processes have become known in which the electrode is supplied with high-frequency alternating current.
The corona apparatuses operating according to this basic principle more or less differ only in the design of the supporting surface used as the counter electrode. Thus, corona apparatuses are known which comprise a central roller and several electrodes, or several supporting rollers associated with a corresponding number of electrodes. Further, the supporting surfaces differ in the materials of their dielectric coverings. Thus, for example, coverings of glass, mica, ceramics, plastic materials, e.g. polyester, special types of rubber, e.g. silicone rubber, and the like are used. Further, the apparatuses are distinguished from each other by the design of their electrodes, which may be in the form of, for example, plate, wire, comb, knife, half-shell, spring, or spindle electrodes.
Finally, the known corona apparatuses differ in the generators used for producing the high-frequency alternating current, either low frequency, medium frequency, or high frequency generators being used.
Other known devices depart from this principle in that the electrode is coated and the supporting surface is uncoated. For this purpose, so called "roller electrodes" are used. Such electrodes are described, e.g., in U.S. Pat. No. 2,864,755 and in Canadian Pat. No. 553,045.
Corona apparatuses of this type have the essential advantage that in case of damage it is simpler and less expensive to replace the dielectric covering of the electrode than that of the supporting surface. The characteristic feature of the construction of such roller electrodes is that a metallic body is used which is in the form of a solid or hollow cylinder and that the shell of the cylinder is covered with a dielectric insulating layer. This layer may be applied to the cylinder by coating or by vulcanization and thus may be inseparably attached to the cylindrical body. More recently, replaceable roller coverings have been preferred, which advantageously use tubular materials which can be pulled over or shrunk on the cylinder.
Electrodes of this type have the drawback that the charged parts form a big mass and that, consequently, electrical losses occur by reflection. These losses result from the large reflecting surface of the electrode, since the entire roller body is energized even though only a fraction thereof is needed for use as the discharge area. These relations become even more unfavorable in the case of apparatuses of relatively large working widths, where the diameters of the roller electrodes must be increased for constructional reasons, e.g. to prevent the rollers from sagging. Since this results in an increased discharge area, the energy density and, consequently, the effectiveness of the pretreating process are reduced. Further, the electrode rollers become hot during the treatment, and cooling them is a problem which cannot be solved simply by rotating the rollers.